Method and system for reducing noise in the transmission of electric signals



Aug. 12, 1952 OTT 2,606,969

' H. H. SC METHOD AND SYSTEMFOR REDUCING NOISE; IN

THE TRANSMISSION OF ELECTRIC SIGNALS Filed Jan. 17, 1946 9 Sheets-Sheet 1 '1 20 0m 81 2% 6 d; 4 E9 20 u:

2o 50 \00 :00 S00 poo'gooo 5,000 {0.000 20,0020

FREQUENCY m CYCLES PER secouo ADJLIST AB LE L .P F L TER f r I (Z;- 3.6" /35 W 2 If I 43 2 J/' 1 ,Q U) $315 8/. /8

IF- 23 CONTROL 2 2/ 2 i4 VOLTAGE '*-H- CONTROL CIRCUIT Hermon Hosmer Scott ATTORNEY Aug. 12, 1952 OTT 2,606,969

H. H. SC METHOD AND SYSTEM FOR REDUCING NOISE IN THE TRANSMISSION "OF ELECTRIC SIGNALS Filed Jan. 17, 1946 9 Sheets-Sheet 2 ADJUSTABLE L.P. FILTER ADJUSTABLE H.P. FILTER Hermon Hosmer Scott ATTORN EY Aug. 12, 1952 H SCOTT 2,606,969

METHOD AND SYSTEM FOR REDUCING NOISE IN THE TRANSMISSION OF ELECTRIC SIGNALS Filed Jan. 17, 1946 9 Sheets-Sheet 3 BROADCAST TRANSMITTE R OR AMPLIFIER CONTROL A VOLTAGE FILTER 1 AMPLIFIE INVENTOR Herr non Hosmer Scott BfW/Wm ATTORNEY Aug. 12, 1952 Filed Jan. 17,

RESPONSE RESPONSE H. H. SCOTT 2,606,969 METHOD AND SYSTEM FOR REDUCING NOISE IN THE TRANSMISSION OF ELECTRIC SIGNALS 1946 9 Sheets-Sheet 4 d c b o i l I 2,500 3,000 lqooo FREQUENCY IN CYCLES PER SECOND /Slope Sdb/octove Slope 6 db octave FREQUENCY IN CYCLES PER SECOND Fig. 6

INVENTOR. Hermon Hosmer Scott BY Z Aug. 12, 1952 H. H. scoTT 2,606,969

METHOD AND SYSTEM FOR REDUCING NOISE; IN THE TRANSMISSION OF ELECTRIC SIGNALS Filed Jan. 17, 1946 9 Sheets-Sheet 5 v /70 A 5? ,5 H7

- CONTROLLED //a 4 cmcun' c 1 0 0 r7 /6 0 o u a CONTROL FILTER CIRCUIT r7 65 rao /7 CONTROLLED I W //6 CIRCUIT g z r r CONTROL CIRCUIT JNVENTOR.

Hermon Hosmer Scott ATTORNEY Aug. 12, 1952 I-I. H. sco'r-r 2,606,969 METHOD AND SYSTEM FOR REDUCING NOISE] IN THE TRANSMISSION OF ELECTRIC SIGNALS Filed Jan. 17, 1946 9 Sheets-Sheet 6 /4 I mv r80 /7 49" F l I 4) 20 N7 AMPLIFIER c I'\ fL I a/ i VARIABLE -GAIN I72 -AMPL|F|ER 'CONTROL T I ER CIRCUIT Fig. 9

g A527 RADIO VARIABLE -GA|N OUTPUT RECEIVER A57 AMPL|F|ER= /8/ H o 6 0 9 A9 VARIABLE -GA|N AM PLlFlER .l

CONTROL FILTER CIRCUIT a INVENTOR.

Hermon Hosmer Scott BY A TTORNEY Aug. 12, 1952 SCOTT I 2,606,969

METHOD AND SYSTEM FOR REDUCING NOISE; IN THE TRANSMISSION OF ELECTRIC SIGNALS Filed Jan. 17, 1946 9 Sheets-Sheet 7 *fi l l wa I I r/a I #8 I IL A I I 7 7; ---fi J3 I 7/ 1 K20? I I I l 84, l 4; l !L /72 l I l J 7 6 L -L Tm, We I KL I I r I I 22 I @T I J CONTROL CIRCUIT IN VEN TOR. I

A TTORNEY Aug. 12, 1952 H. H. SCOTT 2,6067969 METHOD AND SYSTEM FOR REDUCING NOISE IN .THE TRANSMISSION OF ELECTRIC SIGNALS Filed Jan. 17, 1946 9 Sheets-Sheet 8 ADJUSTABLE L. P. FILTER x a p72 W I 80 INVHVTOR. Hermon Hosmer Scorj A T. TOR/V5 Y Aug- 1 1 H. H. SCOTT METHOD AND SYSTEM FOR REDUCING NOISE; IN THE TRANSMISSION 'OF ELECTRIC SIGNALS 9 Sheets-Sheet 9 Filed Jan. 17, 1946 INVENTOR. Hermon Hosmer Scotr.

a I l I i atented Aug. 12 1952 UNITED STATES aeoess'si METHODJANU SYSTEM rofi in inii ii NOISE 1N T I IE TRANSM1SSION 0F ELECTRIC SIGNALS Hermon: H, Scott, Lincoln; .-Mass'., .a'ssignoL; :l iy'" mesne assignments, to Myron-31?; Smith, Com

cord, .Mass., trustee The present invention relates to electric methods and systems, and more particularly -to-methods of and systems for attenuating or "suppressing background noise, interference; and'other spurious disturbances during the transmission, the recording, orthe reproduction -of speech, music, video, and other-signals. Theterm "reducing or its equivalent .w'ill hereinafter be employed generically to include eith'er attenuating or completely suppressing.

Background noise and other extraneous "disturbances, at one-time or another, are'attendant upon practically every form of transmission, recording, or reproducin'gsystem. Theyoccur, for example, during the transmission and the reception' ofthesight and sound channels of television, the presentation of sound-motion-pictures, the transmission and reception of" signals by wire, telephone, cable, or "radio, in"relay-circuits,'and inthe'reproduction of all typesof re-" cording. They have heretofore been "attenuated at the 'expense'oi seriously impairing the fidelity with which the desired signals have been transmitted. The customary practice hasheen' to adju's'tthe frequency 'response characteristic "or'to' restrict "the frequency; range of the system "so'as topr'ovide" what has been" considered to-rbethe best compromise between thefifldelityfolithe transmission and the suppression "oifthe inOlse.

The compromise has not; however, ibeen' satis factory.

To take an' illustration fromphonographrecord reproduction; the interference has consisted'verylargely,of-the annoying surfacenolses caused by the scratch of-Ethestylus -or=needle;: These" scratch noises" usually accompany the high fr'equency components only of "the-repro- "ducedtalk'or music, which high'frequenclescomprise mainly harmonics of the 'lower'frequencies. In recognition of this fact, the oompromisepro-z posed was the elimination of"these high ire- 'quencies altogether. 'I-h-is,- however, resulted in serious -efiects =uponthe brilliance and- 'the nat- "ura-lness*of-themusic.

" The restrictioniipon the fidelity of; the-"repro "duction was particularly--noticeable at the highvolume or high-intensity-levels of the reproduoed signal; at the -"low'-in'tensitylevels, ---the" 'human ear does not seem to notice-the absence-fof the high irequencycomponents ofsound As; moreover," the spurious noises are' very efiectively masked, so as tobe-neaI-Iy-or quite imperceptible to the human ear; whenthe-speechor--m1sic reproduced-f-by=*the-phonograph reoord i'sfloiid-, it {was proposed to attenuatetile high-frequency components 1 at the low olume" levels ;on1y and to permit reproduction-of zthenhighwt-requency range {at the ;:high::levels, either of the hi iheflf quenoy eomponents themselves; or of: thesreprothe ear ;does.not notice::therhighefrequeney-noise components: ,j Elhewproposed modification cannot be availed of-:;genera1ly, thereiorepwhen the :response' to thehilghr:frequencies is @un ace mpanied'rbya signal, at low' frequencies :of; substantial level.

. In 'these-previou'slyi' proposed gsystemsf highlevel rhigh-frequency ,interierenee signals have "been: reproduced; withoutza'ny. attenua -th r of, .:b ecause their very, presence allows ;the;.;sys- 'tem to reproduce: the high-irequenqy; ranges.

' Systems depending :ior their qefieotiveness upon control in accordance wit-hathe =level intheshighfrequency rangezyare fundamentally s'defeotive in ethat the rgreater"- porti'on' vof the control mayr-be exercised? by, the inoise-frequency-- components themselves, z the noise level is ;--high: enough.

- This permits-10f reproductionwin, the-ghigh-frequencyt-rangerwith accompanying spurious noise, when the level: :ofa-theidesired' signal fisrlowi; .and zparti'cularlyisoiwhen there is no nesiredvsignal' at all toibereproduceckin thathigh-ifrequency:range other than the undesired :noise'; During such eriods :of silence or :the iidesired sisnal, evenbe- 'tWeem-notes-or musical phrasesp stheuhigh-ireouency scratch-is wery vannoyingly,,-:heasd,s;and it is all that isrthendieardn- It is yery-zdesirable to. attenuate the veryzaudibletsneedle: match that many accompanyzzthat.,niusimsilenoe .same considerations apply; however-, ,:eiz. e periodswf nonsilenca wherexthe: volume?-.'level-:.-of the reproduced music contains IlOt-BlOlld :lowifrequency components; and whencmostl'ofe therhighfrequency musical: components cexistonlygatglevels too i-lowi to tbe heard;

It :is accordingly; ant-object =of-.t he :presentinvention to provide .:ai:new:iand:-iimproved method of and system fo'ru-educing tback'grountk noise: or other interference during transmission; recordmgg. reproduction, and 1-; Sthe a. like; that shal 1.;,-r

The Y 3 mm practically a high degree of apparent fidelity.

To the attainment of this end, a feature of the invention resides in reducing the response in the high-frequency region, not in accordance with the level of the higher frequencies, and not in accordance with the level of all the frequencies as a Whole, as has heretofore been proposed, but rather in accordance with the level of some of the low frequencies, or the frequencies below the range of the high frequencies that are being controlled. The noise is suppressed when the level of these low frequencies is so low that the signal cannot mask the noise; or so low as to indicate that any probable level of the higher frequencies, insofar as the desired signal is concerned, is below the threshold of hearing. The noise is thus reduced at low-volume levels by reducing the range of the high-frequency overtones that are not normally heard by the ear except at the highvolume levels. Better suppression of noise, with less effect upon the reproduction of the higher treble tones, and with no apparent decrease in the quality of reproduction, is thus attained. If the signal is such that the level of the low-frequency components is high compared to that of the high-frequency components and, particularly,

- the noise components, the described operation, of course, takes place when the level of the signal as a whole falls below a predetermined level.

Though the example above has been given, for illustrative purposes, in connection with the spurious noises attendant upon the high-frequency components of the signals coming from a phonograph record, the invention is not limited to such use. The invention is of general applicabiiity. It may be used to advantage to reduce the noise attendant upon the reproduction of practically any type of system involving recordings -made upon wax, discs, tape, film, wires, etc. It

may be used also to reduce various types of interference common in radio-broadcasting, such as adjacent-channel interference, frequently called monkey-chatter, heterodyne whistles, the effects of atmospheric and other disturbances, and in any other application where noise is a #factor. It is equally. useful at audio, video, and

other frequency ranges.

The invention finds particular application where the characteristics of the receiving or detecting means are not constant, but vary from instant to instant in accordance with variations in the frequency or the amplitude of the signals. Music is an example of a non-constant signal, as it contains various frequency components at different amplitudes, varying widely from instant to instant. The human ear is an example of a receiver or detector the degree of perceptive response of which, far from being constant, varies in a complicated manner with variations in frequency and amplitude. The human eye and photographic emulsions are furtherexamples of ,receiving or detecting means that exhibit similar variable characteristics.

In all such cases, it is desirable, at any instant,

to include, in the frequency range of the transmitted or the received signals, those frequency components only that are necessary for satisfactory transmission or reception in the particular frequency ranges, and to exclude those frequency ranges that would introduce a high noise level. Those frequencies only need to be included that are present at sufficient amplitudes to be detectable by the ear or other detecting. or receiving means. The present invention takes advantage of the characteristics of the ear to eliminate noise by eliminating the response under conditions where the ear response is weak. It also takes advantage of the characteristics of the types of signal most generally encountered by eliminating the response to frequency ranges that are not likely to contain important signal energy.

Prior-art systems have been subject to the further defect that, even when the noise levels, after adjustment for best possible noise reduction, were not sufiiciently high to cause difficulty, the quality of the reproduction changed noticeably as a function of the level of the signal. In some systems, for example, a variable-filter network has been employed, comprising either a resistance and a variable-reactance tube or a fixed reactance and a variable-resistance tube. Such systems provide, at best, only a relatively gradual type of cut-off characteristic, thus affecting frequencies within the audible range, as well as those above.

Most noise, moreover, and, in particular, random noise, such as is caused by tube hiss, shot efiect, record scratch, etc., tends to be distributed, so that substantially equal energy is contained in equal-frequency bands. With noises of this character, the gradual reduction in high-frequency response obtainable with these prior-art systems must, by its very nature, produce serious modification of the high-frequency tones within the range of audibility, if it is also to produce appreciable reduction in noise level.

A further object of the invention, therefore, is not only to control the relatively-high-frequency components of the signals in accordance with the levels of the relatively-low-frequency components, but also to provide that the control shall be an optimum for the purposes of interference reduction.

Another object is to obtain maximum reduction of the spurious high-frequency noises with minimum effect upon the quality of the reproduction of the musical tones.

Toward the attainment of these ends, a feature of the invention resides in the employment of a special type of variable f lter having a very sharphigh-fr'equency cut-off; and, according to a further feature of the invention, suppression of the spurious-noise interference may be effected by varying the cut-off frequency in accordance with the characteristics of the signals.

Prior proposals for eliminating interference have been subject also to further objections not necessary to relate at length. Among them, for example, is the objection that they seriously affect the reproduction of sudden, loud, transient sounds, such as occur to a varying degree in all types of music and speech. This defect is generally caused by the slowness of operation of the control circuits, or by spurious components introduced by the control operation.

For satisfactory noise reduction, it is essential that the system operate with such rapidity that transient sounds such as cymbals be reproduced properly so that the background noise shall not be heard between musical phrases or between individual notesin amusical phrase. Such control action must not introduce spurious noises or altered .in the process of eliminatingthehighfrequency components of the signals at the lowvolume levels. Though thus effectively eliminating the spurious noise, therefore,,the elimination of the high-frequency components may, under some conditions, if audible components are in any way alfected, result in a loss of apparent brilliance of the reproduced signal, and in a loss of balance or symmetry between the high-frequency and the low-frequency responses.

This may be understood by considering, for example, audio-frequency music signals, the relative balance of which is referred to as aural balance. Auralbalance maybedefined as the ratio of the audible energy inthe treble notes to the audible energy in the bass notes; or the ratio of the reproduced high and low frequencies. Those components only of the energy are taken into account, in this ratio, as are of such frequency and amplitude as to correspond to audible tones at the particular levels at whichthey exist.

.Another object of the present invention, therefore, is to provide a novel methodof andsystem for maintaining a substantially constant balanced change in the balance may be objectionable, particularly if the modification seems to vary with the volume level of the sound. The aural balance between the energy in the treble notes and the energy. in the bass notes obtained in accordance with the present invention yields a high degree of apparent fidelity such as to satisfy the ear, even under conditions wherefrequencies within the audible range for the particular level to be reproduced are attenuated to a degree that would otherwise be noticeable. This allows greater suppression of noise with less noticeable efiect upon musical quality than in prior-art systems.

To the attainment of the above ends, a further and important feature of the invention resides in the introduction of a predetermined amount of resonance into the response of the system when the range is restricted. The reproduction of transients, as judged by the ear, tends also to be improved.

The maintenance of constant aural balance so greatly improves the operation that, in general, the only noticeable effect of the use of the noisesuppressing system is the desired effect, namely, the suppression of noise. It also increases the flexibility of the system with regard to the type of signal, listening conditions, etc.

The desired end may be attained, in accordance with another feature of the invention, by suitably varying the characteristics of the transmission or filtering means.

The problem of maintaining balance has also been solved in a different manner by reducing the very low frequencies when the high frequencies are reduced. Both methods may be employed in combination.

Prior-art systems also, have a tendency to be 1 abscissae being plotted logarithmically in terms of cycles-per-second frequency, and theordinates critical with respect to the type of signal, the character of the noise, the listening conditions, and other variables. This is very undesirable where the equipment is to be manufactured for generalsale, to operate undera wide variety of conditions.

A further object of the present invention, therefore, is to provide the system with a degree of flexibility sufficiently marked to reduce the tendency toward critical operation. I

With these ends in view, a feature of the invention resides in controlling the shape of the cut-off characteristic and the resonance effects of the filter in the neighborhood of the cut-off frequency so as to minimize the reduction of theapparenthigh -frequency response of the system, as reflected in the reproduction of transients and in: the brilliance or brightness of the reproduction, even for types of signals or listening conditions thatdo not coincide exactly with the particular types of signals, or the particular conditions for which the system is primarily designed to have optimum characteristics. The apparent-frequency range is thus maintained substantially constant under varying conditions, andforvarious signals.

Other and further objects will be explained hereinafter, and will be more particularly pointed out in the appended claims. I

The invention will now be more fully explained in connection with the accompanying drawings, in which Fig. 1 presents aseriesof graphs representing the frequency-response characteristics of the human ear, and the relative amplitudes and frequencies of the various components present in a typical signal, such as orchestral music, the

being plotted arithmetically in terms of decibel intensity units; Fig. 2 is a diagrammatic view of circuits and apparatus illustrating an embodiment of the invention employing but asingle adjustable low-pass wave filter; Fig. 3 is a similar view illustrating another embodiment of the invention employing two adjustable filters, one

high-pass, thus combining theprinciples of the present invention with a system for controlling the very low frequencies; Fig. 4 is a view similar to Fig. 2 of a modification; Figs. 5 and 6 present v explanatory diagrams. of typical frequency-response curves, with the abscissae, as in the case .of Fig. 1, plotted logarithmically, in terms of cycles-per-second frequency, and with the ordinates plotted arithmetically, in terms of decibels- .units response; Fig. 7 is a block-diagram view representative of the circuits and apparatus of Figs. 2, 3, 4, 9, 10, 11, and 12; 8 isa similar view of a modification; Fig. 9 isa similaryiew the block-diagram arrangement of Fig. 9; Fig. 12

is a fragmentary view similar to Fig. 4, showing a typical alternate means for controlling a filter by means of a vacuum tube; Fig. 13 is a view showing an alternate electric filter which may be used in the systems of Figs. 2, 3 and 4, and comprising a parallel-T network; and Figs. 14 and 15 show further alternate filters comprising two reactance tubes.

Though the method and system described herein are applicable wherever the degree of perception of the signal-receiving means varies with the characteristics of thesignal, and wherein balance is important, th inventiomfor concreteescapee ness, will be explained as itapplies to a typical application, namely, the reproduction of music and similar signals by -a phonograph or aradio.

From the curve I of Fig.'l-,- representing the threshold ofhearing-sensitivity for an ordinary human ear, it appears that the ear is most sensitive in the region around 2,000-cycl'es, and that the sensitivity falls off" rapidly'at both lower 7 and higher frequencies; r

The curve 2- maybeconsid'ered to be representative of the energy-distributioncharacteristics,

- representing different frequencies. These points represent the limits of the frequency range nec essary to reproduce satisfactorily for the listener theparticul'ar-typesof signal at the particular volume or intensity levels. Ifthe level of the curve '2 isredhced equally throughout therange,

for instance, to occupy the position-of the curve 3, the necessary-or perceptible range of frequencies is-seen to become accordingly restricted, from about 70 cycles to about 250 cycles at the lowfrequency portions of the curves- 2 and 3, respectively,-and from-about 12,000 cycles to about 4,000 cycles at the high-frequency portions of the curve 3. If themusic or other signal should have frequency components outside these ranges, they would not be audible.

Most musical instruments, when played softly, moreover, produce a mellower tone than when played loudly, thus indicating the presence of fewer harmonics or overtones falling within the higher-frequency ranges. While the curve 2, as before stated, may become transformed into the curve 3 when the sensitivity of the transmission means is reduced, therefore, it may actuallybecome transformed into the still lower curve l if the reduction of volume or intensity is caused by the orchestra playing more softly. The lower curve 4 shows that thebefore-mentionedlow-levell limit of 4,000 cycles may thus become reduced toabout 2 ,500 cycles or less when the orchestra is playingsoftly; the greater portion of theaudible energy'is below this-2,5'0O-cycle value. Since most music, by its very nature, covers an appreciable volume or intensity range, this characteristic offers the further advantageous feature, therefore,

thatit allows further restriction of the frequency rangeofzthe'transmission equipment at low-voln ne or low-intensitylevels, from about 4,000 cycles to about 2,500 cycles, without seriously im-- pairing the quality, as perceived by the ear. A similar situation exists ior'speech andvocalmusic- The suppression of noise according to the present invention is based upon these and similar factors.-

The curves 2 and-4' may, therefore, be regarded as representative of typical signals involving high :and low volume or intensity levels, such as may be :encountered'in the reproduction of music-and similar signals, the curve 2- being at anaverage volumeor intensitylevel of, say, '63 decibels in.

the region'aroundLOOO't-o 2,000 cycles, which contributes most to the loudness as heard by the: ear, and the curve 4 at a volume or intensity level of approximately 18 decibels in the samerange.- The pointswhere the curves 2 and 4 cross the curve I, at'lO- and 12,000 cycles for the curve 2, an'd 250 and 2,500'cycles for the curve t, respectively, represent the ranges necessary to" reproduce these particular signals satisfactorily and low frequencies up to about 12,000 cycles; otherwise, the reproduction will fail to include some of the audible components of. the music signal represented by the curve 2. When the orchestra plays=sotly, howeven the low-frequency "rangeupto-about 2,500 cycles is-all that isneeded,

as demonstrated-by the curve-4; all the frequenciesabove about"2;500 cycles may then be eliminated without any serious-effect upon-the reproduced music. At intermediate levels-of course,

' intermediateranges are required.

If the system, through other limitations, is not able to reproducesatisfactorily the :entire range up to l2",000 cycles, it may be possible to restrict the high-frequency range to a limit considerably lower than2,500 cycles, at low levels, without further noticeably impairing the quality of reproduction ascompared with high levels;

If the type of signal difiers appreciahlyfrom that represented by'the curves 2 and-'4, however,

' if the listeners acuity of hearingdiffers from that represented by the-curve I, or if the level of reproduction is increased, some of the frequenciesthat are attenuated m'ayliewithin the range perceptible to the listener.

If the transmitted range of the music is less than'the' range of the components-that would be perceived by the ear, anunnaturally low-pitched high-pitched quality woul'd' be imparted to the music, depending upon whether the reproduced signal-is most deficient in the higher the low frequencies, respectively. It is desirable,-therefore, not merely: to reduce noise and othercomponents lying in certain frequency ranges, but

also'to accomplish the reduction in such away as to minimize --any-resulting change of balance that might beperceived by the ear-underconditi'ons varying from the exact conditions as typified by the curves of Fig. 1.

'Asbefore indicated, one'of the objectsof the present invention is to maintain the apparent quality of reproduction approximately constant, even though the signal and other conditions should vary somewhat from those represented by the curves of Fig. l.

I Forsignalsof the character described howevenevenwhen a wide range'can be reproduced,

1 there is noadvantage whatever inreproducing the very-high-frequency ranges that are above the threshold of hearing, represented by the points 'awhere'the signal curves, such as the curves 2: and

l, intersect the'curve I, representing the sensitivity characteristics of the ear. When-reprozducingthe signals represented by the curve 4,

as an illustration, no frequencies in thehighfrequency ranges-say, above about 2,500 cycles, are necessary. Asthe level increases, however, .higher"frequenoies are required for completely 'Lfaithful reproduction until, when the curve 2 is reached, frequencies up to 12,000 cycles are so required. 'This extreme upper limit may be determined by'other considerations, such asthe rereorder, the loudspeakenand other parts of the system.

"When-one listens to phonograph records of orchestral music, however, the background-noise level-is greatly afiected by achange in the highfrequency cut-oil front some high value, such as 12,000 cycles, to a low value, such:as 2,500 cycles. This alone indicates that the noise is at aeoaeea threshold of hearing, and .considerablyabove the amplitude. of many of .the. highrfrequency 001117; It is hence undesirablev ponents in the music. to ..control the-responsewoi the systemin this high-frequency. regionbyl the amplitudeofthe,

high-frequency components inthe signals, "as has beenproposed in manyprioreart systems,.sin'ce these. components willfrequently be constituted. mostly of noise,,which it is desiredtosuppressand.,.the, ,highere-level noise components may be,

considerably louder. than the lower-level. high.- freqllencycomponents. ofv the music.

According. to a feature .of. the. present invention, therefore,v advantage ,istaken ,of...the fact thatthehigh+frequency range, above about .2,500; cycles,,consists. mainly. of harmonics or overtones that are always accompanied by lower-frequency components consisting of fundamentalsthat, are generally ofhigher amplitudes than the highfrequency components. Such fundamentals .as occur in this extremely-high-frequency region. above 72,500 cycles, .approximately three octaves above. middle ,0 and,.higher, occur .only oci casionally in music, .and are thenalmost always accompanied by lower-frequency. fundamentals. Theresponse of the system tov the high-frequency componentsis therefore preferably controlled, according ,to a feature of thev present invention, in accordance. withlevels of. the signal energy. presentinthe lower-frequency components, say, below about 2,.500 cycles. Notonly are the loud spurious noise's,.involving theshigh .frequencies only, not reproduced ,at. the. low-intensity levels of f the sound,..but the frequency 7 characteristics, of, the

sensitivity of the control, means maybe made. to .approximate those :of the human ear, with I theresult that the syst'emresponds .only tothe... highfrequencies under conditions where there is,

suflicient lower-frequency energy -in .thesignal masking effect occurs, in the human ear, for frequenciesbelow the high-frequency range under discussion... There is no need for. control. by the. veryelow-frequency range, as below 250 cycles,-. Where the. ear. again .becomesv relatively .insensia.

tive.

The range .of frequencies referred to as low connection. with tone-control, systems.

music are considered as .very low.

Referring now to Fig. v'7, when .an input. signal,

asfrom .a phonograph pick-up, a radio-receiver,

or another source'l9, of impedance Z1, is applied to'theinput. terminals l and..l6, it is transmitted, by input-dead conductors 50 sand. 5!,

through-a .controlledcircuit 10,. and-by wayof output-lead conductors 80-and18l, to the output terminals I! and I8. Thesemay be connected toany output load.20,of impedance Z2, such 'as a loudspeaker, .anamplifier, or a :radio transmitten:

The controlled circuit 10 may, for example-,-

be .the wave filter5 shown in Fig. 2,.or 69 shown in Figs. 4-and.l2, the wave filters 68 and 390i. Fig 3,v theamplifiers and 4| of Figs. 9 and- 11, or .theamplifiers and liof l ig'rlll. The trans. mission paths provided by, the amplifiers '40 and 4|. of Figs. 9 and..11, andtheampliers 15 and 42:- of Fig. 10. arerespectively connected-in parallel.

.40, to-maskthe noise. It is known that thegreatestv connected "condenser. .102, ."may form, with "t The controlledcircuit' l0 may be designed to pass normally only a restricted range of frequen ciesat loW-volumelevels. This may be expanded to the fullprange that it is desirable to transmit. at high-volume levels; say, from about 7 0 to about 12,000 cycles, or ,fromabout to about 8,000; or more cycles, or any other range, depending upon the application and the limitations'ofase f sociated equipment; The controlled ,circuit 10," may also beldesignedsowthatit shall be; able to attenuate. or, completely suppress, at ,low volume levels, .in, predetermined ratio," .the; 'higheri'range, offiiequencies; say, the higheirequency co ponentsaboye about 2,5001cycles; orjsome sim value, depending upon the application and ptherj factors.

Referring to the. practical'gcircuitfillustrating thejcircuitot 7 thatfis shown in.Fi'g..-, 2',the adjustable low-wpass wave .filter .5 may be} trolled in accordance. with the "level of thefinput signals .to reduce the hig'hrfre'quency components 1 of ftheinput, signal. at, say; above aboutji2,50,0fj cycles. This .control may take'fplaca, inaccojrd ance with a feature of the present invention; at

low-volume levels. of the lowxfrequencies joffthe" input sig'nal;.'say, below about 2,500 cycles. With: inthe term low-volume levels? of coursaii in: cluded' levels of zero volume. The pick -"u 19*, and. the load 20 are shown symbolically as typi source. and load, and may be f-pro'vided jwithjame, plification and thelikawhere necessary. T11. i i

The. wave filter. 5 is, essentially; a loW-pas'sjihalf section comprising .a series arm, shown compris- 55. l The reason why. the, reactance. tube. 8' .f1'1ne.-.

tions as a capacitance will be explained presently,

The anode 4 and thecondenser I l areconne'cted: in parallel Ito the'joutput lead .80 by a .leadQcn-l-.- ductor 25,"and"the cathodeii5'l'fofthe tubeLBQis; connected, to the input lead 5 [and theoutput'lead. 8! by aconductor' 58; As hereinaftenmorelfully described in. connection .with. ,Fig. A,- a, parallel other circuit elements, a complete 1r-,section filter...

7 thus to increase the attenuation Lat, high .'-.fre,-

quencies- The series. inductance l 0. and 1 the. simulated. variable capacitance ,of the. .reactance tube 8.. shown in .Fig.. 2 formnin eifec't, a lowasses stant-lc'. filter; yielding; substantiallyno .at tenu tion withinthe lowefrequency. band,..a'n'd ahignt? degree. of attenuation outside -of that band-f; the high=frequency'range. As withfiltersiztih ,I ing conventional. inductors and capacitors. this. attenuationlis. effected. by. introducing l out-oiie phase components as aresultofthephase shifts. in the circuit elements. These ,out-of-phase components .tendnto cancel} This. will tend to-rpre vent the hig-h=-frequency. components .of thelsig nal voltagev at the input, terminals I 5 Hand; -.l -.S from appearing at the-outputterminals' .l 1 and-,1 l 8. This. is distinguished fromzprior-art. circuitse which have only a.gradually:changingiloss;chars acteristic and 'thatamust, by their very ature cause serious. attenuation. withina desired ,band; in ordertdroduce noticeable noise suppression,

a-b oye that band..-

The attenuation control of a control circuit 6. The control circuit 6 is of such nature as to produce no noticeable effect upon the quality of the reproduc tion; The input side of the control circuit 6 is subjected to the action of the input signal from the signal source l9 through the medium of lead conductors 52 and 53 respectively connected to the input leads 50 and The control circuit 6 is designed so as automatically to shift the response range of the controlled circuit 10.

The output of the control circuit 5 is connected, by a conductor 24, to the cathode 51 and, by a conductor23, through a resistor 12, to the control grid 55. The resistor 12 thus transmits bias voltages from the. control circuit 5 to the grid 55. The grid bias thus developed in the system of the control circuit 6 is therefore transmitted by the leads'23' and 24 to the adjustable filter to impress an adjustable control voltage from the output of the control circuit 6 upon the grid 55. The reactance tube 8 is thus controlled to vary the cutoff of the filter. A more elaborate arrangement which may be substituted will be later described in connection with Fig. 4.

The capacitance H, the resistance 12 and the other elements are so connected as to provide a feed-back network between the plate or output and the grid or input circuits of the reactanoe tube B. This feed-back introduces a phase lead of approximately 90 between the plate voltage and the grid voltage of the reactance tube 8, at high frequencies. The plate or output circuit of the tube 8, with its associated circuits, therefore, as before stated, functions as a capacitance. Equivalent circuits for effecting thi result may, of course, be used. The magnitude of this simulated capacitance of the reactance tube 8 is a function of the transconductance of the tube 8 and this, in turn, is a function of the electrode voltages.

The adjustments are such that the control circuit 6 automatically adjusts the cut-off frequency of the filter 5 so as to attenuate all frequency components of the input signal above a predetermined value, such as 2,500 .cycles, at 'lowvolume or low-intensity levels of the signal, or predetermined ranges of the signal. The spurious noises accompanying the high-frequency components are thus eliminated at low-volume levels with a minimum effect upon the tone quality. The system thu adjusts itself automatically to the signal to be transmitted.

Any vacuum tube, such as the tube 8, moreover, may contain more than one control electrode and, as is well known in the art, separate control electrodes may be used. for the adjustable bias and the feed-back. Depending upon the normal ratio of signal levels .and biases which may be used with practical tubes, moreover, it may be desirable to add attenuation to the circuit, as in the leads 50 and 5|, or gain to the circuit, as in the leads 52 and 53, to provide best operating conditions for the vacuum tube.

Most treble tones, or high musical fundamentals, lie below the typical high cut-off frequency of about 2,500 cycles, and are of relatively low frequency compared to the relatively high frequencies that are attenuated in accordance with the present invention. For the larger part, the relatively-high frequencies that are attenuated in accordance with the present invention are the harmonics or overtones of these treble fundamentals plus a few extremely high treble fundamentals constituting a small portion only of the total energy in the music.

The characteristics of the control circuit 6 should be so selected for each range as to insure the best performance with the intended type of signal.

A filter network I may be inserted in the lead levels higher than the desired components. An

amplifier Hi0 may also be inserted in these leads 52 and 53 to increase the magnitude or the effectiveness of the control voltage, and to provide optimum control characteristics.

The control circuit 6, which controls the transmission of the filter 5, is thus actuated by signals obtained through the filter l. A purpose of this and similar filters is to reduce the sensitivity of the control means in the frequency ranges being controlled, thus to prevent the transmission of high-level noise in those ranges.

The control circuit 5 is shown in Fig. 2 as comprising a series rectifier 9 for rectifying the signal, connected in the control-circuit-input lead 52, and a condenser I4 connected across the controlcircuit-input leads 52 and 53. The condenser I4, which bypasses signal voltages between the conductors 23 and 24, may be located in associated circuits. The resistance of the rectifier S and the capacitance of the condenser i4 provide an R-C filter circuit for reducing the high-frequency components of the rectified signal voltage. A leak resistor I3 is connected in parallel with the capacitance 14. The rectified voltage of the control circuit varies in accordance with the amplitude of the signal applied to the input terminals l5 and 18.

The time constants of the rectifier 9 and the condenser l4 should be so chosen as to allow for rapid expansion of the high-frequency range without introducing spurious noises into the output circuit. The resistance of the leak resistor l3, on the other hand, should be relatively high in value, so that the response of the system shall not contract suddenly, and thus accentuate vibrato or other musical effects. The system should contract at approximately the same rate as the decay of reverberation in normal music, or, when the noise level is high, at a faster ,rate. Proper selection of the time constants and novel features of the controlled circuits allow the operation of the system to follow rapidly individual notes or phrases in music, thus providing a high degree of noise suppression with no substantial effect upon the quality of musical sounds including transients. As a practial matter, it is possible to use time constants of such value that the system range will expand satisfactorily for musical transients, but will not open on single loud clicks, as from a cracked record or static. The condenser 35 is also of importance when the circuit must be controlled rapidly, as will be later explained.

In the case of signals such as those depicted by the curves 2 and 4, where the volume level is always greatest in a range not being controlled, the control will still reside with the signal, rather than with the spurious noise, even thoughthe filter I be not employed, providing that the spurious noise is always at a lower level than 13:3 lowererrequency,componentszof .the.desiredsignal-.-. When-:signals; in the; lower:ranges; :.are-;present, therefore,-.the; controlcircuit- .6 functions "to;- in-; crease-.the range o-f the controlled circuit 10,'-thus1. permitting 'oftransmission ovena Wide frequency; range;

AsiIlustr'ated in Fig; 8, however, a result simir. lanto-thatobtained with the aid. of the filter?! may beiobtained withoutemployingthe.filter 1., The: leads 52 and 53 are shown in Fig.:8 connected-to to outputztenninals I7. and l8,;asindicatedwbythew leads 3S'Land 56: The..c0ntr0'l;1v0ltage cis: then derived, not from the input, but from the output; voltage :of the system, under-:the controLofi the output signal, at the output. level;. The terminalscls l5 and [6 may represent: thev inputand' the termi-J; nals l'l. and IS the output,"-or:vice;versas.

If the leads-36 and 56-are.thus'connectedta they. output terminalsj I. and] 8,-so:as to'efiect control. front-the output circuit, andif. the "control'circuit .20; 6 is sensitive to highfrequencies, the :transmission 'of the high frequencies: will not be reduced 1; untilathelevel: of the hi'gh'ufrequencies, as well as ofthe. lowifrequencies, becomes decreased. The transmission of thehigh frequencies cannot then, 25. however, be increased by. an increase: in thelevel of thehigh frequencies. It canbe increased only? by anincrease in the level of-thelow frequencies; While this operation is not identical with control by.- the=-input circuit, through the'filter='l, it is 30, satisfactory in many applications, and allows-of eliminating the-filter 'Lthus saving parts and cost.

It still has the advantage that the-frequencyrange is not increased byhigh-levelhigh-frtquency noise. With control from the output circuit, and without: employing the -fil-ter 1 l, the frequency range of the --sys-tem' *can expand only when thelowfrequenciesincrease, but will 'contract only when all the frequencies" decreasea The additionof the filter 1 to..the system of'Fig.-8 40. in the-leads 59 andBO would gi-ve operationsimie lar tothat of the system of Fig.7.

At normal-signallevels inthe lower frequency ranges, for the type of "signal: depicted by the curves 2 and 4, the filter l, in :the system of Figs.- 5 2 3', and. 4, is unnecessary to insure'thatthe main control shall-be exercised-. by the lower-ire" quencies; providing that any H high-frequency e noise 'componentsthat may :be' encountered are not so strong as the lower-frequency-components' in thesignal. The filterl'may beeliminated-in the-systems of Figs-p2, 3;and-4alsoifthe input signal is ofsuch character that frequencies within; the-controlled range are normally at. a relatively low level, so that: the main controliisss, by other frequency ranges;

Where-a considerable degreeof :noise suppres sion is required, with a minimum effectupon the' audible quality,.-v there should be substantially i no att'enuation'within therange of frequencies perceived by the: ear, and the attenuation beyond the highest perceptiblefrequeney .shouldbeas great 2 as possible The transmission-cut-offx charac teristicsxshould therefore be assharpxasspossible:- witli apparatus .that; is .-economicallypracticable;

Theconventional RFC or L.-R; tone control of a radio receiver may. be renderedv automatically-.- controllable in response to volume level by means.

of areactance. or variable-resistance. tubeszorother devices; The rate of increase of"attenuation iob,-.rtain'edwith 'a circuit of. this .character,:;however,- approaches six decibels .peroctave :as :a maximum-g; andr theccut-ofi is" .veryrgradual,.-. as:represented by-z-the; curve 172. of -Fig.:;-:6. Because, rofrlthis .graclg ually-changing loss;.characteristicrapplied .to; the.

.-if.-it producesi any appreciable attenuation-of high 1 1'4; high.-frequency responseasuch-operationds often unsatisfactory; By. -reason of; its. very-"ginature WhBIlzfidjHStQd-EQI'PIQOVidGra reasonable-degree; noise suppression, it:1changes,-.;- noticeably; the-.5 quality;;ofi zthe reproduced musiegat. low-volume levels; reducing; -the: brilliance or: :brig-htness; by;-, 1 attenuating.;.; high-frequ,ency:: tones 5: within father, range ofrtheearc;

Thevgraduahcut-ofl illustrated-by; the am frequency noiseg will, also: produce; atzaudiblezf quencies.,-.;noticeable; attenuationofthe signal :fre quenciesathus;-:causingga idull lifeless characte in.the-reproduced;music;. Such a-";Ci1 011it:-ZWhe producing, for instance, anmattenuation .ofz-i2 decibels,at 8,000 cycles, which is a typical uppe frequency ;limit for phonograph :records;z;would.: providealso .an attenuationiof the .order.-of;8 ;orr r more; decibels at 2,000 cycles,; and 3. ,or. more -decibels at 1,000v cycles. The range from11,000,-to;;;- 2,000 cycles is Where the normal ear is most sensie tive,;and it also contains a large amountzof the-- soundenergy in orchestral and other'typesaofr music,= including both important fundamentals; and overtones.- With such a-system, the reduc.-- tioniin noise will obviously berelativel small, and the effects upon the quality ;of reproduction of tone balance'very noticeable,-

Any reduction in response in thisregion, where the ear is. most sensitive, is extremely-noticeable::- and, as before explained, it definitely, 10W81S2l3h8i1': balanceof thetreble tones to therlower-rnotes This" results in causing the previouslyementioned lack .of-brilliancevor brightness, andv an .iunnat-w ural dullness-or boomy quality in;the;;repr0duc.':=-

tion.

Any attempt to, improve-the ,qualityby decreas ing 1 the amount of noisesuppression restores the:- difiicultyof' the; presence ofspurioustnoisez; This ;is because, as before explained, .when:using:- an:1

R-C, an L-R, or an equivalent-circuit.therattenuation; increases; sovery;v slowly-i 1' with t, fife- J quency Many .gnoise phenomena, such as the scratch" noise on-phonograph records, .tend to .have equal. amounts'of energyzin equal-bandwidths: For? this reason, further to improve the: apparentw fidelity of a system of this'type, anoptimumre-e I duction-in noise, with the least impairment of musical quality, may be obtained 1 with :a sharp 9 cut-off. characteristic, as represented :by the; curves-9 and k of Fig. 6. The curvera'demon-zstrates that a relatively sharp cut-:off. m may bexobtained in ,a correctly. designed ,reactanceatype wavefilter, such as is illustrated in Figs: 2, 3;;and

4.- The curve k represents, a similarcut-oif, but witha resonant peak-.nh

The curve i is'characteristic. of a -reactance-s typev wave filter matched to theoretically correct: loadimpedances; or otherwise connectedtoimpedances having a certain amount of, resistance art-dissipation. Thecurve k is characteristic of a similar filter operating atimpedances deviating:-

from conventional matched values;- It willlater be pointed .outthat. connecting; a relatively lowa impedance across thelzinput terminalslJE ands-l6 provides: more rapid control of the. filter: This; tends :also to produce a curvesuch:: as is:-::indi+* catedat. kr-insteadof the curve'y'.

Resonance. may be. obtained; forrinstance;- by

operatinga filter, likethe filter 5 of-.Fig.r2,i.from:;

a relatively low-impedance 9. The eresonance-tisi further; increased .by. operating.:the filter "into'ia high impedance, as across the:leadsyfimand ZBI'.:- The; rate; of; increase .of: attenuation;;:for both curves :1 and k is greaterthan six decibels per octave. These curvesgi and It also provide less apparent decrease in the high-frequency response, because of the sharp cut-offer the resonance.

The sharp cut-01f of the curve i reduces the balance of the music less than the gradual cutoff represented by the curve h. A resonant type of cut-off, as illustrated by the curve It, however, introduces a certain amount of resonance, which would generally be considered undesirable at high-signal levels, but which is not objectionable at low-signal levels, and tends to compensate for the 'loss' of brilliance caused by attenuating the highfrequencies, besides improving the reproduction of transients.

The curves 7' and It, therefore, are preferable as cut-off-ohara'cteristic curves for the type of system described, not merely on the basis of greater suppression of noise, but also because they provide better apparent high-frequency response. In this type of system, the cut-off functions at low-signal levels only, and the high-frequency range expands as quickly as possible upon application of a higher signal level. Because any artificial brilliance caused by the cut-on characteristic is present only at low levels, and the cut-off or resonant frequency shifts rapidly upon application of a loud signal, a more realistic reproduction of transients is provided than in a system as typified by the curve it, even though the latter system does not provide'so good noise suppression. The disclosed circuits therefore provide both better noise suppression and better apparent fidelity than prior-art devices having relatively gradual cut-01f characteristics. system of the present invention, furthermore, is less critical in'regard to the typeof signal, the listening level, the listeners acuity of hearing, and other variables usually encountered than prior-art systems.

'Ifthe same listener always heard the program over the same range of levels, and the orchestra always produced exactly the same sound spectrum, it would be a simple matter to determine what frequency ranges could be attenuated or suppressed, without affecting the quality noticeably. In any practical application, however,

many variations prevail. The usual type of signal, for instance, does not remain constant. Any given signal may deviate from the characteristics represented by the curves 2 and 4, the listening conditions may be different, and an individuals hearing may vary from the average. All listeners, again, do not have equal acuity of hearing. The total sensitivity of the system, from microphone to loudspeaker, moreover, may be varied, or the volume range of the program maybe contracted, thus changing the range of listening levels.' These factors may result in a condition where, sometimes, the transmitted frequency range of the music is less than the range of the components that would be perceived by the car. This would impart an unnaturally lowpitched or high-pitched quality to the music, depending upon whether the reproduced signal is most deficient in the high or the low frequencies, respectively. It is desirable, therefore,

not merely to reduce noise and other components lying in certain frequency ranges, but also to accomplish the reduction in such a way as to minimize-any resulting change of balance that might be perceived by the car under conditions varying from the exact conditions as typified by the curves of Fig. l. I

For listening levels and music corresponding The v exactly to the levels depicted in Fig. 1, therefore, a sharp cut-oif characteristic in the filter is all noticeable reduction in the high-frequency re- I sponse.

In accordance with a feature of the present invention, however, apparent constancy of aural balance between the energy in the treble notes and the energy in the bass notes may be maintained under such conditions by using a filter having a predetermined degree of resonance in the neighborhood of cut-off, or having resonant circuits operating below critical damping. By reason of this resonance or less-than-critical damping in the neighborhood of the cut-off frequency, an apparent intensification of the response is provided in the region of cut-off, through actually increasing either the amplitude of the components or the duration of transient components in this region, or through a combination of both effects. The net result is to compensate for any reduction in the apparent energy of the treble tones, as heard by the ear, in a manner alternative or supplemental to the possible reduction of the very low frequency components. The apparent balance between the treble and the bass notes in the music is maintained by increasing the energy in part of the treble range when the energy in a still higher portion of the treble range is reduced in order to reduce the noise.

The curve k of Fig. 6, for example, shows, at n, a rise'in response with respect to the curves h and 1', thus indicating an increase in amplitude for a particular range of frequencies. Both curves 7 and k exhibit, with respect to the curve It, sharp cut-01f characteristics m and n, indicative of a reactance-type filter that operates through the medium of resonance effects. These effects, if the circuit .is less than critically damped, tend to increase the amplitude or the duration of transient sounds in the frequency range involved, thus increasing the stimulus to the ear and, in effect, intensifying, such transient sounds. The term intensifying may be employed, therefore, to designate increasing the amplitude, or lengthening the duration of, the signals, as through resonance,- thus enhancing the efiect of the signals in the particular frequency range.

The present invention, through the use of a filter 5 two or more of the arms of which may each have a reactive impedance, provides a sharp cut-off sufficiently steep so that the attenuation of the high-frequency components of the signals shall increase, above the high-frequency limit,

at a rate greater than about six decibels per -through the condenser I l. ta'nce of the tube 8 which, in accordance with -inusic arising 'outoithe control function; improves the reproduction of "transients.

Aural 'balanceis restoredto'la certain degree,

therefore, 1 by resonance in the filter, which'in- 'tens'ifiesthe response justbelow the cut-ofi. Although this region is r'eierre'd to-a's-a low=irequency region, it low-onlyin comparison with the high frequencies that are reduced. f'Actually,"as .before explained; the higherirequencies in the Lrelatively low-frequency band represent high treble tones up to three or more octavesa-bove middle C. By accentuatin'g these tonesjust-belo-w cut-off, compensation to a largede'greeis efiec'ted for attenuation of the higher frequencies.

.=Theinvention is shown in Fig.3 embodi'ed in a .system for reducing not onlythe high-frequency components, but also the very-lo w f-re'quency vcomponents, for reducing low frequency noises,

or for maintaing aural balance-by alternative or supplemental means. This is a very useful com-' bination. The simple filter of Fig. -2 is replaced by two filter '68 and 39-: theformer, -lo-w-'pass,

for reducing the high-frequency components, as above 2,500 cycles, as in the:sy'stem of Fig.2; and thelatter, high-pass, for reducing the very low-frequency components, as below -250-cycles.

The practical advantage of this is that'the lowpass filter" 68 that attenuates the higher irequenciespas will be more fully described hereinafter, may be made to function much more rapidly than the highpass' filter 39, which controls the'lower frequencies, Without introducing noticeable spurious components into the signal in theoutput ofthe systern as the result ofthe control operation. This improves the reproduction of transients.

The output conductors H5 and Il6-of the low- ..pass filter 68 lead to the input of the high-pass filter 39, and the output ofthe high-pass filter-39 is connected by the output conductors ao-ands I to the output terminals i1 and i8. H

The input connections of the low-pass filter 68 are similar to those of the low-passfilter 5 of'Fig. 2. The outputc'onnectiens of the control circuit 6 to the low-pass filter '68 are 'also substantially the same as in the system of Fig. 2. The connections of the reactance tube 8 of: the, low-passfilterrfiii, howevenare of a somewhat different type ,from those-in Fig. 2, embodying the so-called Miller effect.

.The lead conductor 25 is connected directlyto the grid 55 and not, as in the system oI-Fig -Z,

The input capacithe Miller efiect, isa function ofthetranscom ductance of'the tube 8 is, in this case-increased k'by the addition of the capacity of theigrid-plateconnected condenser H. The capacitor Hincreases the grid-plate capacitance of the-tube 8, thusproviding a larger variable capacitance than would be obtained merely by usin'g'the' internal capacitance of the tube itself. .The simulate'd capacitance is varied, as in the system of Figsz,

by; varying. the grid bias.

The presence ofan additional inductance 59' between the 'conductor25 and the lead conductor l l51converts the filter 68 into a full T section. 'I'his'is shown merely for the sake of variety,and to illustrateanother of the many possible filter configurations. The filters 68 and 3S may be'reversed in position, or the input and'output circuits interchanged.

The input connections of the control circuit iotFig. v3 are shoWnsubstantiallythe same as those of :Fig.- ,2. The control circuit B ee Pia- 3,

filter 65' for the; control voltage 'appli'ed from the control circuit-i6 to thelhi'gh-pass filter 39. The .-'fi1ter:65 is 'showrncomprisin'g a-shunt capacitance filter 65 zpermi'ts of-slower control bf the lowfrequencyrangeby the high-pass 'filter39 than of the high frequency-rang'e' byth'e low-*pa'ss'-iilter asia resultofthe controlaction.

:liksthatroi Fig; egrmay exercise: such control --over the': lowspa's's filter: 68 itofcause the reduc-v tion l'of thetuhighetrequency rcomponentsqioi the T i'riputi's'ignalisay, above about 2,500" cycles, fitrbhe hi'gh-vo'lume levels :of. the low-frequency 0.0mponent's "of 'a "signal: such as" that representedz by the curves 2 and 4 of Fig. 1.

- l The wave filter: 3 Bier-Fig. 34s essentially ahighpass half-sectioncomprisingia .se'ries capacitance 29 :6ppcsite sidesptwhichs'are connected; tothe leads and I I5, and a variable'simulated :shunt inductance connected-across the leads "80 'alidiia l The :shunt inductance "c'o'mprises an inductivereac'tanc'e' tube '3-3.,fprovided with a ifeederback network connected-between its anode or platetfl 'andits'control-gridelectrode '6 I. i'The feed-hack 1 circuit, -intro-ducing aphase lagiofrapproximately n;:between* the laterivcltage and thef gridtvolt- .age-of the rea'ctance tube "3 3, at low? irequenc'ies cbmprises a series+connected inductance .i3fland ."blocking condenser. 32 connected across itsfanode orlplate fifland its COIItrOIF-gIld electrode 1'6 It; and a resistorx3'l The-plate or :output circuit :er the tube"33., withiits associated circuits, therefore, functionsas an inductance. Equivalenticircuits for efiecting the result may, of course,:be used. The anode stand the series connected inductance -30 and blocking condenser 32 :are connected. in

parallel, to the lea'd 80by a conductor 62,:and the cathode .63 of the tube-33 is. connected to'the output lead 8| by a conductor 6 I Inthe system of Fig. 3, therefora'the filters 68 andjas'are both'ofthe all-'reactanc'e type. The reactance tube 8 simulates avariablezca-pacitance, and .the reactance tube 33 a variable inductance.

The resistor 3 I is connected :between thev control circuit- -6--.,and the grid-6 I by. aconductor 27,

' similarlya tosthe connection of the resistor. I 2 to the gridsii5 rby theconductor 23. Th ricondu'ctor 28 corresponds similarly; toitheconductor 2 4. ii The control ;circuit -6' thus controls f-the f high-pass filters!) :similarly tothea 'controlo'f the low phase The :output' ail-seam or the control-circuit fi to the filter .39 may embody an-"a-dditibna'LR-C '26 .and "a series resistance 66. Though not absolutely necessary,the; presence of this additional '68. -This also reduces a tendency to introduce spurious components into the high-passffilter 39 The control circuit *6 of Fig.- 3 is shown-as of a slightly'idifierent type,'in' that the rectifier 9 -18 connected in parallel with S. the parallel-con- :nected condenser 1 land resistor I 3 instead or in series therewith. interchangeable, anumay be used in thesyst'ems These control circuits are of any of the figures, with or Without extra am- ."The variable-low pass filter 69 shown in 4,

68 'of'Flgs; 2 and 3; has iniprovd characteri ice.

The series arm of the filtercontaining the in- -ductor |0 tuned bymeans; or aware-mercannected. capacitor'38- to providea parallel-resonant circuit. This provides an 'm derive'd' filtr "1th a corresponding fixed point of high unsubstant-ially infinite= attenuation above tut-bit ofthe normal transnii'ssion range, where the-cape tor 3c: resonates with the inductor ID. This represented at a in Fig. 5. The attenuation of the filter above cut-off becomes thereby improved.

By eliminating the ,tuning condenser 38. in the 1 series arm, the fixed point of high attenuation may be eliminated, if unnecessary forthe particular application, as in the. systems of Figs. 2

and 3.

A second inductor 34, connected in series with theconductor25, and therefore in series with the variable reactance of the reactance tube 8, forms 1 a series-resonant circuit with the simulated ca- .pacitance of the reactance tube 8. This Provides a point of high or substantially infiniteattenua- This variable point may vary in a predetermined relationship .withrespect to the cut-off frequency. It may vary from a low value, such as 2,500cycles, to.- a, high value, such as 8,000 cycles or higher, depending upon the characteristics and the limitations of other parts of the system. A resistor 31 may be connected in the output circuit .of the reactance tube 8.

Each of these fixed and variable cut-oif points assures an extremely sharp cut-off. This filter, therefore, provides unusually sharp cut-ofi'and unusually high attentuation above cut-off, allowing of rapid control.

The three curves of Fig. 5 illustrate a typical operating range for the filter of Fig. 4, the curves 2 and 9 representing typical cut-off characteristics for maximum and minimum attenuation, determined by the amplitude of the input signal, and the curve. ,1 illustrating an intermediate cutoff characteristic. .The cut-offzcurves e, f, and 9 may, of course, haveother shapesin the cutofi region, such as are representedsby'm andn in Fig. 6. For each variable cut-off frequency of the filter of Fig, 4, there is a corresponding variable point of very high, or substantially infinite, attenuation, represented at b, c, and cl, re-

spectively, caused by. theseries resonance of the inductance. 34 and thecapacitanceof thereactance tube 8, and-limited only bythe loss of tenuation, as at a. This circuit, therefore, functions as a double-m-derived filter having anexceedingly sharp variable high-frequency cut-off characteristic with a very high degree of attenuation above cut-off, and it may have a predetermined amount of resonance in the region of cut-ofi, thus to increase the response in that region. This type of filter may be varied very rapidly, in accordance with the control voltage or current, without introducing spurious components cf'the; control voltage intothe output .circuit of thefilten 1 a As shown clearly in Fig. 5 andin accordance with the previous; discussion concerning Fig. 1 and the desirable characteristics for a system of the character described, it will be noted that the circuit as shown in Fig. 4 is capable of producing high degrees of attenuation above a certainfrequency such as 2,500 cycles while maintaining substantially constant transmission at low .fre-

' 'quencies.

[- -The fixed point a of high attenuation should generally be placedabove the normal: operating range. For reproducing ordinary shellac phototeristic.

20 graph records, which contain frequencies up. to approximately 8,000 cycles, 9,000 cycles is a good value for thispurpose. Ifthe system is to be used also for the reception of amplitude-modulated radio broadcasts, 10,000 cycles, which is the difference-between adjacent channels, is a desirable value, as the 10,000-cycle attenuation will reduce also the heterodyne whistles. For'prcgram sources involving higher frequencies, of course, this fixed attenuation point a may beset at a still higher frequency.

The said fixed and variable points of high attenuation are points at each of which the attenuation reaches a maximum with respect to adjacent higher and lower frequencies, and are determined by series or shunt resonance in .one of the filter arms. If the resonant circuits causing the points of high attenuation had no loss whatever, or no resistive component, the attenuation at these points would be infinite.

Any impedance connected to the input leads 50 and 5|, such as the pick-up l9, Or any interposed amplifier or network, may have an 'impedance sufiiciently low so as to provide a certain amount of resonant rise in the filter characteristic below cut-off, or less than critical damping for the resonant circuits in the filter. This condition'is shown by n in Fig. 6 as previously described in connection with Fig. 2. This tends to provide a certain amount of compensation to the ear for the cut-0E of the higher frequencies under conditions where frequencies that might be audible may be attenuated, as may occur upon the application of a sudden loud transient.

The resonant efiect is improved :by connecting a high impedance across the leads and 8|. This condition of resonance may exist at highsignal levels only momentarily, while the cut-oil. characteristic is shifting to a higher frequency.

The resonant condition is therefore not apparent to the ear as such.

, The condenser I02 increases high-frequency attenuation by forming a full TF-SBCtiOH with the other filter arms. This-condenser may be used in any of the circuits shown.

A condenser 35 maybe connected in series with the inductance 34 or with the lead 25 in series with the reactance tube 8. It may be used for blocking or tuning purposes only, or it may be utilized also to improve the control characteristics of the filter. If used, it will, in effect, form a high-pass filter with the inductance [0. This filter will shunt to the low-voltage side of the system, through the input circuit of the filter, any spurious low-frequency-audio components of the control voltage that may reach the control grid 55 from the control circuit 6, and that might otherwise have appeared at the output terminals H and [8.

.As shown in Fig. 4, the grid 55 and the condenser l are connected in parallel to the output lead 80 through the series-connected inductor 3t and capacitor 35, and the cathode 51 of the tube 8 is connected to the input lead 5| and the output lead 8| by a conductor 58.

The action of this low-pass filter is particularly efiective when the input circuit is connected to a relatively low impedance; and this, as already explained, will also improve the cut-off charac- Rapid control of the circuit may thus be attained without thumps or other audio components of the control voltage applied to the grid 55 that might appear in the output circuit.

Typical alternative methods of connecting condenser 35 are shown in-Figs. 2 and 3. In each 2'! "case; the condenser prevents low-frequency surges appearing in the plajteof the reactance t"ubet as a result of=the control "action-from being transmitted to the output terminals I'I and -'I 8.

The control circuit of Fig. 4 is showrr including extra amplification I and filtering I to provide optimum control characteristics. =Ihismay be in accordance with the characteristics ofthe human ear, cr-for any desired range or. frequencies. :The control circuit may pass frequencies outside of the controlled range only, 3 thus "providing improveddiscriminationagainstnoise;

7 Theinput filter network- I embodied in the'sys- *tem' cf'Fig. 4 is shown comprising a" condenser 91 and a resistor 2| connected in series in the frequency response. A series condenser 94" in the input-circuit of the c'ontrol circuit 6, serving' as a blockingcondenser'between the amplifier I00 and the control circuit 6, may'function also to reduce the sensitivity at lowfrequencie's. The functionof the shunt capacitors 22 and 95 is to reduce the-sensitivity of the control'system at very high frequencies, so that-the control shall xbe essentially in accordance with' frequencies where the ear'is most sensitive. The function of the series capacitors 9 1 and 94 is to reduce the responseat very' low frequencies below the cutoff frequency, where the ear is relatively insensitive, and most of thesig'nals may consistmf a rumble, "hum; or other components. These -condensers I4, 92, Q-I'and- 94' may be used in any of the circuits shown. Essentially the result of 'the control of the circuitmay be to approximate to the sensitivity characteristics of the'ear forthe "type of signal'to be reproduced. j

More elaborate filter and ampliffierflcircuits may be used-where warranted. r All the tubes are operated within-their normal The control circuit 6 utilizes an accompanying output filterfor filtering the control voltage comprising a series resistor 9 If and a parallel capacitor 92', to provide better-filtering of the rectified con- "trol voltage, thus increasing further'the speed of "control; without introducing spurious thumps into the output. r

The following typical" values of the elements 'e'mployedin the system of Fignl'have been found 'to be very eiiective, inpractice: v

Capacitor 38.4,. ."1.=l0 micromicrofarads,

' J approximatel Capacitor I I 1:60 micromicrofarads,

' approximately Vacuum tubes B and 913- 6SJ '7 Resistors Hand 9-] 1 megohm- Capacitor I- i- 0.01 microfarad' Capacitor- 9L- 0.003 microfarad v I,

'22 Capacitor 94 .300 micromicrof'arad's Resistors ZI, 99, and I04 500,000 ohms:

Capacitors 9'! and I02- 500 micromicrofara Capacitors 22 and 96 200 micromicrofarads Rectifierts";;; 6H6. 1 Source-impedance'Z; 35,000 ohms, approxi- V: l t ly- I. Load impedance Z2 0.5 megohm, approximately.

other variations of. constant-kl 'me deriveilattice-type' andotherfilters, and various. other networks, :such. .as parallel-5T,- bridge-T, etc which provide characteristics: similar; to filters,

may'zalso be used; provided that theyzcontrol the -frequency :response 2 inthe mannerfdescribed;

Practically any combination of high-pass and low-pass filters may be useditogetherr Thel'owminating'im pedanceJZz on the midi-shunt side,

of the filter. The various filter half-.sections-lmay beconnected together? to form T or 1r sections.

Fig-.13 shows onetof the: manylpossible -arrangements incorporating a parallel-T network in place of the parallel-resonant circuit comprising theinductor Ifl'and the capacitor 38 in'the series arm. Alternative connections for the reactance tube 8 are also shown, combining themductance 34 of Fig. 4 with:- the reactance tube of Figs-2 and,3.i 'The filter 69 ofvFig's. 13; 14

and 15 may be substitutedzdirectly forthe filter 5. of .Fig'. 2, the fi1ter 68 of Fig. :53 or the-filter One of" the -parallel Ts of Eig; 132i shown comprising series resistors 21 0 and i2'l'I and 'azcapacitor= 2l5 connected in shunt between them. The other T. is shown eomprising'seriescapacitors .2 I 3 and 214. and. aresistor: 2| 2 connected in shunt betweenthem; -A shunt capacitor I02 is shown by dotted lines to indicate that it omitted, if desired. l This. .parallel-T'network of Fig; 13, as is well may.v be

known.--may provide a null pointer a point of high attenuation in the transmission characteristic. TypicaLbutnot exclusive values for such'a null are: I 1

Capacitor 2I3=capacitor 2I4= /2 capacitor 2Y5 Resistor 2I0=resistor 2| I"=2 resistor 2 I2 The null point; with this combination, will occur .at-th frequency where the reactance-oi the capacitor 2I5 equals in magnitude the resistanceo'i the resistor 202.

Such a network has a phase shift passing through zero at the 'null point 'andof opposite above and below the null a complete section. The connections tothe reactancetube Bfare similar to those in Fig. '13, but with the alternativeplacement ofthe condenser 35; The

'reacta'nce'tube I08 is similar to reactance-tube 8 of Fig. 2, with the reference numerals augmented by I00. This reactance tube I08 and th'e condenser I02 of-Fig. I3 perform a 'similar'function, that' is; to provide additional high frequency attenuation; The-reactance tube is-preferablyto 

